1. Field of the Invention
The present invention relates to an image forming apparatus using an image carrier such as a photosensitive drum and an intermediate transfer belt. In particular, the present invention relates to an image forming apparatus that uses a gear-speed reduction device configured to drive an image carrier.
2. Description of the Related Art
Conventionally, an image forming apparatus uses a gear-speed reduction device, to drive rotation of a photosensitive drum or an intermediate transfer belt.
For example, in an image forming apparatus using an intermediate transfer belt, power of a drive source such as a motor is transmitted to a driving roller moving the intermediate transfer belt, while a speed is reduced by a plurality of gears.
In a gear-speed reduction device, it is usual to avoid using an integer ratio, as a ratio between the numbers of teeth of a plurality of gears. This is because when the integer ratio is used, a certain tooth of a gear unevenly wears out, by repeatedly coming in contact with the same tooth of a counterpart gear to mesh with.
The above-mentioned wearing out will be described using an example of a speed reduction device having a speed reduction ratio of 4:1 (an integer ratio). This speed reduction device includes a gear A having 80 teeth and a gear B having 20 teeth, as illustrating in FIG. 1. The gear B is driven by power, and the gear A follows by meshing with the gear B. When a speed reduction ratio, namely, a ratio between the numbers of teeth of the gears, is an integer, a tooth B-1 regularly comes in contact with a tooth A-1. Repeated contact by the same tooth causes uneven wear, so-called imprinting, in which the shape of the tooth B-1 is transferred to the tooth A-1 as illustrated in FIG. 2.
Here, an adverse effect due to the occurrence of the imprinting in an image forming apparatus will be described.
FIG. 3 illustrates a velocity fluctuation when the gear A rotates. A velocity fluctuation component of one turn and one tooth occurs corresponding to the precision of the gear A. Here, the number of teeth of the gear A is 80, which therefore creates a waveform in which sine waves of 80 teeth are superimposed on a sine wave corresponding to a velocity fluctuation for one turn of the gear A. Similarly, FIG. 4 illustrates a velocity fluctuation when the gear B rotates. The number of teeth of the gear B is 20, which therefore creates a waveform in which sine waves of 20 teeth are superimposed on a sine wave corresponding to a velocity fluctuation for one turn of the gear B, and this waveform occurs successively. The speed reduction ratio is 4:1. Therefore, the gear B makes four turns for a single turn of the gear A. When these two gears rotate while meshing with each other, the velocity fluctuations form a composite wave, becoming a velocity fluctuation illustrated in FIG. 5.
When the speed reduction ratio is an integer ratio, the imprinting occurs due to the contact between the same teeth of the gears meshing with each other, as described above. When the shape of one tooth of the gear B is imprinted on the gear A, a velocity fluctuation component of the one tooth of the gear B is further superimposed on the velocity fluctuation of the gear A. Rotation irregularities occur when the two gears mesh with each other. The amplitude of the rotation irregularities occurring over time is larger than the amplitude in an early stage, as illustrated in FIG. 6. This increase in amplitude, i.e., an increase in velocity fluctuation, causes an adverse effect such as banding.
Meanwhile, there is an image forming apparatus having a speed reduction device in which an integer ratio is intentionally used. This speed reduction device drives a photosensitive drum that is an image carrier, or an intermediate transfer belt that is an intermediate transfer body. For example, as discussed in Japanese Patent Application Laid-Open No. 1988-113477, it is known to use an integer ratio as a speed reduction ratio on purpose in a speed reduction device that drives an intermediate transfer belt. This method is used to improve out of color registration in order to cancel a velocity fluctuation of a belt surface by a distance between photosensitive drums.
As described above, when the ratio between the numbers of teeth of gears is an integer ratio, imprinting occurs which causes uneven wear on a gear. Therefore, when an integer ratio is intentionally used, a resin material such as polyoxymethylene (POM) with high slidability is widely used.
Meanwhile, recent speed enhancement of image forming apparatuses has enabled use of a medium having a large grammage. At the time when the medium having a large grammage enters a secondary transfer part, a load on a speed reduction device driving a photosensitive drum or an intermediate transfer belt becomes greater than the load in the normal time. A load fluctuation when such a medium enters transfer part, may cause elastic deformation of a gear, leading to a velocity fluctuation of the photosensitive drum or the intermediate transfer belt, thereby creating a shock image. Therefore, gears of the speed reduction device have been expected to be rigid.
However, when the speed reduction device is configured of only metal gears with high rigidity to avoid the shock image, gear-to-gear contact noise becomes large, which makes the image forming apparatus less comfortable for a user. Therefore, a combination of a metal gear and a resin gear is used. However, if an integer ratio is used for the numbers of teeth of gears as discussed in Japanese Patent Application Laid-Open No. 1988-113477, and a combination of materials which are greatly varying in longitudinal elasticity is adopted to form a combination like a metal gear and a resin gear, an imprint on the resin gear having a smaller longitudinal elasticity becomes conspicuous. Therefore, an adverse effect such as banding of the image cannot be avoided.